A transverse excitation (TE) or transverse excitation atmospheric laser is a gas discharge laser with transverse excitation of the laser gas. The first TE lasers were first built in the early 1970's and consisted of arrays of resistively ballasted metal pins arranged in rows extending along the length of an optical resonator. They were direct current (DC) discharge devices that required excitation pulses to be restricted to very short periods of time, typically a few microseconds. The duration of excitation pulses was limited because the large volume glow discharge conditions that existed at the beginning of an excitation pulse would constrict and develop into arc discharges after only a few microseconds.
Unlike glow discharges, arc discharges fail to pump the entire volume of the laser, and cause distortions and power fluctuations in the output beam. Traditionally, these problems were minimized by turning off excitation pulses before "glow-to-arc transitions" could occur. Another way by which the formation of arc discharges was kept to a minimum was by limiting the pressure of the laser gain medium. The natural consequence of this limitation, however, is a limitation of laser output power.
The primary challenge concerning the improvement of TE lasers has thus been the production of large volume, long duration, glow discharges in the highest pressure laser mixtures while minimizing the occurrence of glow-to-arc transitions. Because there are numerous factors that affect the frequency and severity of glow-to-arc transitions, this challenge has been a difficult one.
Parameters affecting the laser discharge include the electric field distribution in the discharge space, free electron distribution within the laser gas mixture in the discharge volume, the electrode surface conditions, and the gas temperature distribution in the discharge volume. The primary objective for the production of large volume glow discharges has been to ensure the uniformity of each of these parameters.
Arc discharges tend to form within the laser discharge volume at locations in which the plasma distribution is not uniform. Such non-uniformity is more likely to occur when the electric field applied to the discharge volume is not uniform. The plasma/electric field interaction is not one-way, however: plasma non-uniformity at one location can grow, concentrate, and eventually distort the electric field around it. This interaction between the applied electric field and the plasma can lead to the collapse of the volume glow discharge into an arc discharge. After some investigation, it was found that the frequency of such glow-to-arc transitions could be diminished by the use of profiled electrodes, instead of the arrays of pins used in the first TE lasers.
Profiled electrodes were designed to produce a large volume discharge with a uniform electric field. They were designed to minimize distortion of the electric field in the discharge volume due to the effects of the fringing fields that exist at the finite boundaries of the electrodes. The most commonly used electrode profile is known as the Rogowski profile.
The increased discharge uniformity offered by profiled electrodes comes at a cost. The electrodes are expensive to manufacture, and yet, like conventional electrodes, suffer damage when arc discharges occasionally form. Furthermore, profiled electrodes, alone, do not prevent arc formation.
Arc formation still occurs because when a voltage pulse is applied, glow discharges do not naturally form throughout the discharge volume even between two profiled electrodes. Typically, a free electron will induce a breakdown at one location within the discharge volume which quickly degenerates into an arc discharge.
To further prevent such arcs from developing, volume pre-ionization may be used. Pre-ionization occurs, for example, when radiation such as ultraviolet (UV) light is used to ionize gas throughout the glow discharge volume prior to application of the electric field. In effect, pre-ionization uniformly fills the entire volume between the profiled electrodes with free electrons so that no one particular electron has an opportunity to produce an arc discharge.
Although the generation of pre-ionization radiation typically requires radiation sources and additional electronics that increases the laser's cost, the combination of pre-ionization and profiled electrodes can significantly reduce arc discharge formation. But the combination fails to entirely prevent it. Consequently, other factors which affect the glow-to-arc transitions must be addressed.
One such factor is the uniformity of the surface conditions on the profiled electrodes, particularly the cathode electrode. Since TE lasers are DC discharge devices, the secondary emission properties of the cathode have an influence on the current distribution flowing through the glow discharge volume. If a particular spot on the cathode surface favors the enhanced conduction of discharge current, then the discharge will tend to concentrate at that spot, and in turn, the electric field will become distorted and a glow-to-arc transition will form.
The enhanced conduction at one particular spot may be caused from any anomaly on the surface of the electrode. The anomaly on the electrode surface may be caused by, for example, dust or oxidation.
The enhanced conduction at any particular spot caused by an anomaly on the electrode surface modifies the electric field within the laser's main discharge region, and leads to a glow-to-arc transition.
Therefore, the surfaces of the profiled electrodes must be polished, cleaned, free from anomalies, and conditioned by operating the electrodes at reduced pulse energy levels until the electrodes operate without developing the arcs.
Another factor that affects the surface conditions of the TE laser electrodes is the ability of the electrode material to resist the formation of hot spots as a result of the arcs. Arcs have the capability to concentrate enough energy in one particular spot on an electrode surface to pit the surface. The resulting electric field is non-uniform in the region of the pit, and leads to additional arcing at the same location, perpetuating the arcing problem. The severity of the problem increases with the laser's repetition rate, but is present even at rates as low as 1 Hertz (Hz). Therefore, the profiled TE laser electrodes not only have to be polished extensively, but must be made from a high temperature metal or material, such as nickel or stainless steel, that will resist arc surface damage.
Yet another factor that can lead to a glow-to-arc transition in a TE laser is non-uniform heating of the laser gas. As the temperature in a particular region in the laser gas is raised in relation to the surrounding regions, the relative ionization rate in that region will increase, and as a result, the discharge will become non-uniform.
As described above, once a discharge non-uniformity is established, the electric field becomes increasingly distorted and the glow-to-arc transition process rapidly occurs.
To overcome discharge non-uniformity due to increases in the temperature, a technique was developed of rapidly flowing the laser gas through the space between the profiled electrodes in a direction transverse to the optic axis of the laser. This technique allowed TE lasers to operate at pulse repetition rates of thousands of Hertz.
The methods of suppressing the glow-to-arc transition process described above were successful to the extent that the need for ballast resistors in the circuit of TE lasers no longer existed. As stated above, the early TE lasers used an array of resistively ballasted metal electrode pins. The ballast resistors attempted to uniformly distribute the current between the electrode pins, but resulted in a lack of discharge uniformity that prevented efficient laser operation.
The above described methods are expensive, and a TE laser capable of suppressing most glow-to-arc transitions must employ all of them. Consequently, what is needed is a low cost TE laser capable of suppressing glow-to-arc transitions.
Prior U.S. patents have addressed the issue of overcoming the problems of creating a uniform discharge in a TE laser. For example, U.S. Pat. No. 3,986,139 discloses a TE laser that incorporates a radioactive source to provide a partial ionization of the gas medium in the electrode area to provide a uniform pre-ionization.
U.S. Pat. No. 4,905,251 discloses a TE laser utilizing a resistive electrode assembly with pre-ionization; however, the patent does not adequately disclose how the resistive electrode is made. The suggested chemically inert semiconductor material suitable for a sealed laser is not commercially available and generally does not exist.
Barium titanite, uniformly doped, and formed in large sheets, is not commercially available and, assuming it is possible to fabricate, would be a very expensive electrode material and therefore cost prohibitive.
U.S. Pat. No. 3,743,881 discloses a laser utilizing a bulk material with a resistivity to compensate for changes in electrical potential in response to a change in current density; however, the patent does not address other factors that initiate the glow-to-arc transition, such as maintaining uniformity of the applied electric field.
U.S. Pat. No. 4,166,986 discloses a laser utilizing a plurality of individual ballast resistors; however, the array of pins creates local distortions of the electric field, which is followed by a glow-to-arc transition.
One of the primary concerns in fabricating a resistive electrode in a sealed TE laser is the choice of material for the resistive electrode, which most often chemically reacts with the laser gas. The chemical reaction between the resistive electrode and the laser gas changes the laser gas composition, and as a result, the laser gas becomes ineffective as a laser gain medium.
The selection of resistive electrode material for profiled metal electrode TE lasers is limited by surface properties of the electrode material. The resistive electrode material should have uniform secondary emission properties to avoid hot spots on the cathode, such as the hot spots that are clearly visible in the discharge photographs shown in T. A. Johns and J. A. Nation, A Resistive Electrode, High Energy, Transverse Laser Discharge, Rev. Sci. Instrum., Vol. 44, No. 2, Pg. 169.
What is needed is a low-cost TE laser capable of suppressing glow-to-arc discharges.